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Nuclear Fission and Nuclear Energy Production
Published in Robert E. Masterson, Introduction to Nuclear Reactor Physics, 2017
In addition to prompt neutrons, there is another type of neutron that is also produced. These neutrons are called delayed neutrons. Delayed neutrons are produced in much smaller numbers than prompt neutrons are, but they can play a very important role in the control of a reactor—particularly as the power level changes. As the name implies, delayed neutrons are not produced immediately after a fission reaction occurs. On a nuclear time scale, they are produced long after the parent nucleus has been destroyed. Delayed neutrons come from the fission fragments that are left over from the fission of the parent nucleus. These fission fragments are usually “neutron rich” and they have to eject one or more neutrons to return to a stable energy state. The normal progression of events is shown in Figure 7.17.
Fundamental Nuclear Processes: Scattering, Fission, and Absorption
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
Secondly, smaller numbers of less energetic neutrons are produced much later from the by products, or fission products, of the fission process. These additional neutrons are called delayed neutrons, and delayed neutrons normally appear between one tenth of a second and 100 seconds after the fission products are produced. (The exact timing is a function of the type of fission product, and not all fission products are radioactive enough to produce delayed neutrons). The average kinetic energy of a delayed neutron is about one third of that of a prompt neutron (see Chapter 7). Otherwise, the properties of delayed neurons and prompt neutrons are identical.
Point Kinetic Diffusion
Published in S. Chakraverty, Sukanta Nayak, Neutron Diffusion, 2017
The neutrons emitted within a short interval of 10−17 s of the fission process are called ‘prompt neutrons’, which is about 99% of the fission neutrons. The remaining neutrons are delayed in their emission in the fission process itself and are known as ‘delayed neutrons’ (Hetrick 1971, Lamarsh 1983, Stacey Weston 2007).
A Novel Hybrid Deterministic and Monte Carlo Neutron Transport Formulation and Algorithm (tRAPID) for Accurate and Fast 3-D Reactor Kinetics
Published in Nuclear Science and Engineering, 2023
Valerio Mascolino, Alireza Haghighat
The TFM method4,8 is an extension of the steady-state FM method upon which several of RAPID’s algorithms are based. The main differences are that the TFM coefficients need to be generated as a function of time. In addition, there is a need to separate the generation of prompt neutrons from the generation of delayed neutrons for two main reasons. First, the delayed neutrons and prompt neutrons appear with timescales that differ by several orders of magnitude. In fact, while prompt neutrons are generated as a result of fission in a timeframe of around s, and are considered instantaneous from the point of view of the neutron transport problem, delayed neutrons come from the delay of delayed neutron precursors (DNPs) and appear on a much longer timescale, spanning from to s depending on the decay constant of the parent DNPs. This process is demonstrated in Fig. 1. The second element of difference is that the delayed and prompt neutrons may have significantly different energy spectra. As such, the probability that a fission neutron will induce the next generation of fission neutrons can vary significantly for prompt and delayed neutrons.
Multiphysics Coupling Methods for Molten Salt Reactor Modeling and Simulation in VERA
Published in Nuclear Science and Engineering, 2021
Aaron M. Graham, Zack Taylor, Benjamin S. Collins, Robert K. Salko, Max Poschmann
The second reactivity concern is that of delayed neutrons. In any reactor, delayed neutrons play a very important role in reactor operation and safety as they effectively extend the neutron lifetime, and thus the amount of time operators have to react, by orders of magnitude. If reactivity insertions are less than the total reactivity worth of the delayed neutrons, the result is a slower, more easily controlled transient event. However, a large reactivity insertion that surpasses the worth of the delayed neutron can result in nearly instantaneous transient events, referred to as a super prompt critical transient. While there are natural feedback effects that quickly help to mitigate such transients, they can still be dangerous and must be avoided. In a solid-fueled reactor, the delayed neutrons are emitted with effectively the same spatial shape as the prompt neutrons. This is because the fission products that emit delayed neutrons, called delayed neutron precursors, do not move significantly in the fuel rods prior to their decay. MSRs do not have this advantage since their liquid fuel moves the precursors before they decay and emit the delayed neutron, with some of the longer-lived precursors emitting neutrons outside the reactor core. Because of this, there is an effective decrease in the total number of delayed neutrons in the core, which results in a decrease in the margin between critical and super prompt critical. Thus, quantifying the movement and decay of the delayed neutron precursors has important safety implications.
Revolutionizing LWR SMR reactors: exploring the potential of (Th-233U-235U)O2 fuel through a parametric study
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Mohamed Lkouz, Ouadie Kabach, Abdelouahed Chetaine, Abdelmajid Saidi, Taoufiq Bouassa
The motivation behind investigating (Th-233U-235U)O2 fuel is the recognition that while (Th-233U)O2 fuel offers numerous advantages over traditional UO2, its utilization in LWRs has revealed certain limitations. One notable limitation is the lower delayed neutron fraction associated with U-233 compared to U-235. This lower delayed neutron fraction can pose challenges in reactor power control since delayed neutrons play a significant role in determining the reactor’s neutron population and overall power level. Additionally, the moderator temperature coefficients obtained from (Th-233U)O2 fuel are less negative, or even positive, due to the contribution of the epithermal fission resonance cross-section of U-233 at higher neutron temperatures (Raj and Kannan 2022). This characteristic implies that as the moderator temperature increases, the reactivity of the reactor will also increase. Such a trend can have implications for maintaining stable and controllable reactor operations. To address these limitations and reduce the radiotoxicity of discharged UO2 fuel, a partial solution is proposed, which involves the addition of U-235 to (Th-233U)O2 fuel. This approach aims to improve the aforementioned issues and enhance the overall performance of the fuel. By incorporating U-235, the delayed neutron fraction can be increased, aiding in more effective reactor power control. Furthermore, adjusting the fuel composition can result in more negative moderator temperature coefficients, promoting stable and controllable reactor behavior even at higher moderator temperatures.